Conventionally, a motor employed in a compressor has a rotor and a stator placed radially outside the rotor. The rotor has a plurality of slots arranged side by side in the circumferential direction, and one magnet is provided in each of the slots in the circumferential direction of the rotor (refer to JP 2003-32921 A). The slot is set larger than the magnet in order to fit the magnet in the slot, and the magnet is movable in the slot in the circumferential direction of the rotor.
However, as a consequence of a trial to incorporate the conventional motor into a sealed vessel together with a compression element to constitute a compressor, there was a discovery of a problem that sounds and vibrations were generated from the motor. With regard to this, the present inventor discovered that the sounds and vibrations were generated by a lubricating oil in the sealed vessel, which entered the slot and made the magnet easily move and collide with the inner surfaces of the slot.
That is, since the one magnet is provided in each of the slots in the conventional motor, the magnet moves in the slot reciprocatingly in the circumferential direction of the rotor due to the magnetism of the stator.
In short, the magnet is consistently pulled rightward and leftward in the circumferential direction of the rotor by the magnetism of the stator, and the magnet vibrates in the circumferential direction of the rotor in the slot due to the out-of-balance of the rightward and leftward forces.
In concrete, when one slot 111 is located in a specified position with respect to the stator 120 as shown in FIG. 9A, electromagnetic attraction forces by the stator 120 are exerted on the one slot 111 (the rotor 110), and an attraction force (arrow E1) exerted on the left-hand side of the one slot 111 is larger than an attraction force (arrow F1) exerted on the right-hand side of the one slot 111. Therefore, an attraction force pulled leftward is exerted on the magnet 112 in the one slot 111 as indicated by arrow G1, and the magnet 112 collides with the inner surface on the left-hand side of the slot 111.
When the rotor 110 rotates by a prescribed central angle, as shown in FIG. 9B, an attraction force (arrow E2) exerted on the left-hand side of the one slot 111 becomes smaller than an attraction force (arrow F2) exerted on the right-hand side of the one slot 111. Therefore, an attraction force pulled rightward is exerted on the magnet 112 in the one slot 111 as indicated by arrow G2, and the magnet 112 collides with the inner surface on the right-hand side of the slot 111.
In FIGS. 9A and 9B, a coil wound around the teeth of the stator 120 is omitted in illustration. Moreover, a gap between the slot 111 and the magnet 112 is illustrated larger than the actual one for better comprehension.
As described above, the magnet 112 collides with the inner surfaces of the slot 111 many times when the magnet 112 vibrates in the slot 111, and sounds and vibrations are generated from the rotor 110 by the collision of the magnet 112 with the inner surfaces of the slot 111.